I currently have a 55-gallon bow front aquarium with an adult RES, and a Tetra decorative in-tank filter for up to 55 gallons. The water level prescribed by the filter is 4-6 inches, and I have it right to the top of the six-inch mark. The filter is working fine so far and my turtle uses it as a basking spot (with the help of another rock as a stepping stone).
Still, it seems like a lot of water space in the tank, and I would like to make the water deeper. Obviously just adding water would submerge the filter and negate its usefulness, but what if I put something under the filter? If the filter was sitting a few inches off the bottom of the tank, I could add that much,more

